I seeded the lawn with Tall fescue 12 days ago. We tilled the existing soil, added more soil and compost, seeded and lightly raked the seeds after spreading. I could see grass on the 6th day and it has grown almost 1.5 inches tall. The problem I am having is that the grass is all dense in few spots and then few spots are almost barren. What did I do wrong? I did spread the seed evenly using the rotary spreader. What can I do now?
At this point I would do nothing to disturb the other areas.
Wait until the lawn is at least 4-6 inches, then give it the first mow. Then inspect the patchy areas and see if the seeds are there, washed away, or maybe the new soil washed into other areas and nutrients need to be applied to the ground.
After inspecting, reseed the areas and do not mow lawn again for 3 more weeks.
Tall fescue is highly adaptable to wet & dry conditions, so most likely watering won't be the issue even in germination periods.
Wait few other weeks, and eventually over seed again.
At this stage humidity is the relevant factor: seeds and initial stem have very tiny roots, so were the seedling found more water in the few millimeters, they tend to growth earlier. Just shadow or humidity on air on the morning could make such spots.
Another possibility is that the flow of water over the soil, either from watering or rainfall, moved the seeds. You could try to reseed the bare sections now, but you'd probably have to step all over the new seedlings to do it, possibly killing them. So, I'm not sure it's worth it. Might as well wait and re seed the bare patches in the spring.